从平均值计算总和

Question

I have a table with the following attributes; inv_num , inv_amount

I want to create a query that displays both inv_num and inv_amount and also the average of the inv_amount as avg-inv and the difference of the inv_amount-avg_inv

So far I'm able to get the average but I cant seem to figure out how to get the difference between the calculated average and the inv_amount.

SELECT  inv_num, 
        inv_amount, 
        (select avg(inv_amount) from invoice) as avg_inv
from invoice
; 

Answer 1

I recommend using a JOIN operation to an inline view. For example:

    SELECT i.inv_num 
         , i.inv_amount
         , a.avg_inv
         , i.inv_amount - a.avg_inv   AS diff
      FROM invoice i
     CROSS
      JOIN ( SELECT AVG(t.inv_amount) AS avg_inv
               FROM invoice t
           ) a

The inline view (aliased as a ) returns a single row, with column avg_inv . That row gets matched with all rows returned from i , so avg_inv is available to use in expression in the SELECT list of the outer query.